This invention relates to thermal printers, and more particularly, to a new and improved high resolution thermal printer capable of both donor printing (i.e., the print head heats a thermally responsive donor media to transfer a print image to a receptive printing media) and direct printing (i.e., the print head heats a thermally responsive printing media itself). The new thermal printer of this invention has higher accuracy and resolution of print than possible with previous printers.
Thermal printers have been used in a variety of applications. For instance, thermal printers have been common in the facsimile printing industry for some time. Thermal printers use thermally responsive printing media, that is, printing media which responds to heating above a threshold temperature by changing color. The thermal printing operation generally involves transporting a printing media past a linear array of individual heating elements. By carefully controlling the heating conditions, each heating element can cause rapid coloring of the immediately adjacent portion of printing media if activated, but will no coloring if deactivated. As the printing media passes over the print head, the heating elements are selectively activated and deactivated to create an image.
Donor thermal printers are printers which use a thermally-responsive donor material to print onto the printing media. The donor material may have a wax or ink-based substance which melts when immediately adjacent an activated heating element. The melted wax or ink may then solidify or be absorbed on the printing media to form a dot of an image. Similar to a ribbon on a conventional typewriter, the donor media is generally transported past the print head by a separate transport system.
Thermally responsive media (both direct and donor) is generally monochromatic, making full color thermal printing difficult. Color printing has been performed using four-color donor media. The four-color donor media may have individual segments or panels of the four subtractive primary colors (cyan, yellow, magenta and black --"CYMK"). Color printing in this way requires four separate printing passes of the entire image by the print head, each pass using a different colored donor media panel. In between monochromatic passes, the image on the printing media is reversed past the print head, such that a subsequent pass may print an additional color directly on top of the previous image. Accurate placement of the image is necessary for proper printing of subsequent colors on top of the previous image. In part because of difficulty in accurately aligning the image for subsequent passes, color printing has been performed with thermal printers only for low resolution applications. A more accurate placement system is necessary for color thermal printing with high resolution.
Thermal printers have generally been previously used only for low resolution applications such as 50-1.00 dots per inch ("dpi"). Because of the low resolution of thermal printers, thermal printers have not been capable of producing clear text and graphics of high image quality, and have been unsuitable for a number of applications requiring higher resolution. In particular, resolutions of 300 dpi or higher are necessary for high image quality graphics. Resolutions in excess of 900 dpi are typically desired for high resolution applications such as printing offset masters to be used on a printing press. Thermal printers have previously been totally unsuitable for such high resolution images.